Physical inactivity has dramatic health consequences and is reaching epidemic proportions. In our recently completed intervention study, we examined the mechanisms of adolescent physical activity behavior change in a controlled exercise intervention. Though successful in terms of improving physical fitness and physical activity, the study found no support for any of the proposed psychosocial mediators of behavior change. The currently proposed study extends our findings to date by testing a novel biobehavioral model of physical activity behavior drawn from recent theoretical and empirical work. The study will test the following hypotheses: Primary Hypotheses: 1. Adolescents with a more positive affective response to a bout of exercise at baseline will be more likely to sustain or increase their level of physical activity over time. 2. Adolescents with greater intrinsic motivation to exercise (i.e., exercise enjoyment) at baseline will be more likely to sustain or increase their level of physical activity over time. 3. There will be an interaction between acute exercise-associated affect and intrinsic motivation, such that among those with low intrinsic motivation there will be a stronger association between acute exercise induced affect and physical activity participation over time. Secondary Hypotheses: 4. Adolescents with greater left frontal brain activation relative to right, as determined via EEG, will be more likely to respond to a bout of exercise with a positive affective shift. 5. Adolescents who feel competent when exercising, derive a sense of social connectedness from exercising and/or experience autonomy related to exercise will be more intrinsically motivated to exercise. 120 female and 120 male adolescents will complete a resting EEG to assess brain activity asymmetry, and will perform 2 30-minute exercise tasks at moderate and hard levels of intensity, respectively. Affective response to exercise and intrinsic motivation to exercise will be assessed using standardized questionnaires. A one-year follow-up will examine changes in physical activity participation since the baseline testing.